Polyethylene has desirable properties that have helped to make it the highest volume polymer manufactured. Polyethylene can be made in different processes in order to give different properties. Known families of polyethylene include high density polyethylene (HDPE), linear low density polyethylene (LLDPE), and low density polyethylene made using high pressure reactors (LDPE). Within these broad classes many variations exist resulting from different types of reactors (for example, solution, slurry, gas phase or high pressure reactors) or from the use of different catalysts (for example, Ziegler-Natta, constrained geometry, metallocene, and bis-biphenylphenoxy type catalysts). The desired application requires a careful balance of rheological properties which will lead a person of skill in the art to select one type of polyethylene over another. In many applications, such as blow-molding and blown film applications, melt strength of the polyethylene is a key parameter.
The melt strength is a practical measurement that can predict material performance when submitted to elongational deformations. In melt processing good elongational viscosity is important to maintain stability during processes such as coating, blown film production, fiber spinning and foamed parts. The melt strength is related to the number of molecular entanglements of molten polymers and relaxation times of each molecular structure, which is basically dependent on the overall molecular weight and the number of long-chain branches in relation to the critical molecular weight.
Melt strength directly affects several processing parameters such as bubble stability and therefore thickness variation during blown film production; parison formation during the blow molding process; sagging during profile extrusion; cell formation during the foaming process; more stable thickness distribution during sheet/film thermoforming.
This property can be enhanced by using resins with higher molecular weight, but such resins will generally require more robust equipment and more energy use because they tend to generate higher extrusion pressure during the extrusion process. Therefore, properties must be balanced to provide an acceptable combination of physical properties and processability.
The use of highly long-chain branched polymers such as LDPE to increase melt strength or a specific catalyst system that incorporates a high level of long-chain branching into the polyethylene are other alternatives to enhance material processability during extrusion. However, while some properties are improved, high levels of long-chain branching can hurt other properties.
Currently, when increased melt strength is desired, the most common approach is to include peroxides to crosslink the polyethylene. For example, U.S. Pat. No. 5,486,575 improves the properties of a polyethylene resin prepared from a chromium catalyst by using an organic peroxide. U.S. Pat. Nos. 4,390,666 and 4,603,173 use peroxides to crosslink a polyethylene blend containing high and low molecular weight components. U.S. Pat. No. 6,706,822 uses peroxides with polyethylene having a broad molecular weight distribution to reduce melt swell. U.S. Pat. No. 5,486,575 uses peroxides with polyethylene prepared with chromium catalysts. While some properties can be improved by crosslinking with peroxides, there are issues with this approach. The radicals produced can interact deleteriously with other additives. It is difficult to predict the effect of crosslinking on rheological properties. Reported results vary significantly from resin to resin, even when the resins are produced using similar catalyst technologies. Peroxides add an extra component to the composition, and they require careful handling and storage, which adds to the cost.
Moreover, if the crosslinking begins too early in the process, gels will form and these will be carried forward into the final product. Gels are generally undesired for various reasons, including deleterious effects on mechanical and electrical properties and diminished aesthetics, and are generally avoided or at least minimized to the extent possible. In reactive extrusion the desire is to postpone or delay the onset of crosslinking until the initiator is thoroughly admixed with the polymer and the polymer is ready for extrusion.